1. Field of the Invention
This invention relates to an exhaust gas purification system of an internal combustion engine, particularly to an internal combustion engine exhaust gas purification system that uses a selective-reduction type NOx reduction catalyst to reduce NOx (oxides of nitrogen) in an oxidizing state in the exhaust gas by use of HCs (hydrocarbons) as a reducing agent.
2. Description of the Prior Art
One of the known selective-reduction type NOx (oxides of nitrogen) reduction catalysts that decompose NOx in an oxidizing atmosphere in the exhaust gas is the catalyst taught by, for example, U.S. Pat. No. 5,326,735 and No. 5,487,268, which comprise iridium and an alkaline earth metal carried together on a substrate (monolith) composed of at least one material (substance) selected from among metallic carbides and metallic nitrides. Another is the catalyst taught by U.S. Pat. No. 5,402,641, which is an NOx-absorbent comprising of platinum (Pt) or like precious metal borne on a substrate (monolith) and referred as an absorbent catalyst.
Regarding the first-mentioned catalyst, U.S. Pat. No. 5,357,749, for example, teaches a system for purifying exhaust gas by using this type of catalyst and regulating the NOx constituent concentration and HC constituent concentration in the exhaust gas to a prescribed ratio.
Regarding the latter-mentioned catalyst, U.S. Pat. No. 5,402,641 teaches a system provided with a catalyst comprised of an NOx absorbent such as an alkaline earth metal oxide. When the air/fuel ratio is lean and the temperature of the exhaust gas (or the catalyst) is high, the system intermittently or continuously controls the air/fuel ratio to the stoichiometric air/fuel ratio so as to suppress SOx contamination of the NOx absorbent decomposition catalyst and enable regeneration thereof when contamination occurs.
Recently, however, the move toward leaner air/-fuel ratios, as seen in lean-burn engines and direct injection engines (in which fuel is directly injected in the engine cylinder), has created a need for higher NOx constituent purification performance in an oxidizing environment.